LCD devices are among a group of flat panel display devices that have been developed and are being widely used in notebook computers, monitors, televisions, spacecrafts, aircrafts, and the like.
The major components of an LCD device are generally described as an LCD panel, a driving circuit unit, and a backlight unit. The LCD panel includes a thin film transistor (TFT) array substrate, a color filter substrate attached to the TFT-array substrate and spaced apart by a predetermined distance, and a liquid crystal layer formed therebetween. A polarizer is attached to an outer portion of each of the two substrates.
The driving circuit unit includes individual circuit devices on a printed circuit board (PCB), and related components.
The backlight unit includes a light emitting lamp, several optical sheets, a supporting mold, and related components.
In an LCD device, the LCD panel displays an image by controlling the amount of light passing therethrough, and the driving circuit unit applies several signals transmitted from the system to the LCD panel to thereby to control the signals. The backlight unit serves as a light emitting unit for evenly irradiating light to the LCD panel. However, it is inefficient to use a backlight unit having a large thickness, weight, and power consumption. Therefore, development work is continuing to provide an improved backlight unit that solves these problems.
An LCD panel can not be used in a place having no light due to a non-spontaneity thereof. In order to solve this problem, backlight units that uniformly irradiate light onto a surface to be displayed have been developed. The backlight unit serving as a light source of the LCD device has to emit light having a high intensity of illumination with minimum power consumption. Also, the backlight unit converts linear fluorescent light into planar light by irradiating light to every surface of the LCD device with the same brightness.
Backlight units can be generally categorized as a direct type, a side type, or an edge type depending on the position of a light emitting lamp. In the direct type backlight unit, light is irradiated from a rear surface of an LCD panel to a front surface of the LCD panel. In the side type backlight unit, light is irradiated from a side surface of a light guide plate and transmitted to the front surface of the LCD device via the light guide plate. In the edge type backlight unit, light is irradiated from one side surface of an inclined light guide plate and transmitted to the front surface of the LCD device via the inclined light guide plate.
The side type backlight unit includes a light emitting lamp for emitting light, a lamp cover for covering the light emitting lamp, a reflection sheet for reflecting light transmitted to a rear surface of an LCD panel into a light guide plate, a light guide plate for scattering light at surfaces of small glass beads included in dots formed at a lower surface of the light guide plate and passing the scattered light, a diffusion sheet for preventing the dot patterns printed on the light guide plate from being directly introduced into an operator's eyes, a prism sheet for focusing light that has passed through the diffusion sheet and thus having a drastically decreased brightness, and then for increasing the brightness of the light, a protection sheet for preventing the LCD device from an external impact or foreign materials, and a mold frame for supporting the components.
The side type backlight unit that emits light to the entire surface of the LCD panel using a light guide plate has a low brightness since the light emitting lamp is installed at an outer surface of the light guide plate and light passes through the light guide plate. The light guide plate is required to have a high optical design and good processing techniques to obtain a uniform brightness.
The direct type backlight unit is mainly applied to an LCD device having a large screen and high brightness. Recently, a direct type backlight unit having a light emitting device (LED) as an optical source has been developed. A conventional direct type backlight unit having a light emitting device (LED) as an optical source will be explained with reference to FIGS. 1 and 2.
FIG. 1 is a schematic view illustrating a backlight unit for an LCD device in accordance with the conventional art, and FIG. 2 is a plane view illustrating a rear surface of a light guide plate to which a diverter is attached at a portion corresponding to a light emitting device in the backlight unit for an LCD of FIG. 1.
Referring to FIG. 1, an LED array portion having a plurality of light emitting devices (LED) 21 with a certain gap distance is fixedly installed on an array supporter 13, and the array supporter 13 is coupled to a lower cover 11. A reflection sheet 15 formed of aluminum is provided between the light emitting devices 21. In a backlight unit using a light emitting device sensitive to heat, a metal core printed circuit board (MCPCB) is used as the array supporter 13 for heat emission, which differs from a backlight unit using a cold cathode fluorescent lamp (CCFL) and external electrode fluorescent lamps (EEFL).
A light guide plate 23 is arranged above the LED array portion. Also, as shown in FIG. 2, a plurality of circular diverter 25, formed of a reflective material or having a prism structure, are attached to a rear surface of the light guide plate 23 at positions corresponding to the plurality of LEDs 21. The diverter 25 is used to prevent light emission in a vertical direction and in order to implement color mixing. When power is applied to the LED, an optical beam corresponding to approximately 20% of the total is emitted from the LED in a vertical direction and an optical beam corresponding to approximately 80% of the total is emitted from the LED in a horizontal direction. Accordingly, the diverter 25 guides the optical beam emitted in a vertical direction into a horizontal direction.
A diffusion plate 31 and a plurality of optical sheets 33 are arranged on the light guide plate 23, and an LCD panel 41 is arranged on the plural optical sheets 33.
In the conventional backlight unit, as power is applied to the plural LEDs 21, light is emitted from the LEDs 21 in a horizontal direction and in a vertical direction. The light emitted in a vertical direction is reflected by the diverter 25 and guided into a horizontal direction. As the result, light is uniformly distributed on the entire surface of the backlight unit.
In the conventional backlight unit for an LCD device, an LED used in a direct type backlight unit is a high power side type LED, and light emitted from the LED passes in a lateral direction rather than in a vertical direction. When light emitted in a vertical direction is not shielded, a spot is generated and thus the display quality of the LCD device is degraded. In order to solve this problem, a light guide plate having a printed diverter is used in the conventional art. However, since the conventional diverter does not completely shield light emitted in a vertical direction, a display level is more degraded than in an LCD device using another optical source. Furthermore, since an additional sheet such as a light guide plate is required, a fabrication cost is increased and a mass productivity is lowered. Also, since the diverter has to be attached to the light guide plate in a number proportional to the number of the LEDs, the installation is complicated and the installation cost is increased.